The present invention relates to a transmission and control system.
Such a transmission and control system are known from the non prepublished international application number PCT/EP 99/07000, and in general from EP-A-0.451.887. Continuously variable transmissions of these kind for transmission of force fully rely on friction between transmission components and represent a relatively new transmission technology when compared with conventional kinematic transmissions utilising mechanically interacting gearwheels, the development of which for automotive applications started about one century earlier. For safely, i.e. non destructively and lasting operation of such continuous variable transmissions it is a pre-requisite to generally slipless perform the transmission of power, particularly where the frictionally engaging transmission elements, as in the current case, are produced in metal, i.e. of comparable surface hardness so that mutual scratching at contacting surfaces can occur. Amongst others because the contacting surfaces are hardened only to a limited depth of a supervised layer, it is highly important to prevent damage at the contacting surfaces of pulley and belt.
In the early days of the known transmission the clamping force in a pulley required for realising for the transmission of an actual torque to be transmitted was in creased by an additional amount of safety force. This amount of safety was added to all values of pinching force required to transmit any desired torque to be transmitted, within the range of possible torques transmittable by the relevant transmission.
The above mentioned amount of safety added to the required amount of pinching force, could at the largest torque transmittable by the relevant transmission be expressed as a safety factor as was demonstrated by a new transmission design of applicant described in the application xe2x80x9cHigh torque CVT P930, design and test resultsxe2x80x9d at the occasion of the 1997 Imech-E congress in London. It was demonstrated that this safety factor could be used for a so called optimised safety throughout virtually the entire range of torques to be transmitted by a relevant transmission without damage occurring to the transmission. The actual nominal safety factor found practicable for optimising the pinching force applied appeared to be 1.3, which contributed to a significant increase in transmission efficiency as compared to the earlier commonly applied amounts of safety force which compared to a safety factor of between 2 and 2.5 when calculated at the maximum transmittable torque by a relevant transmission. It was indicated to be necessary to control the clamping force as close the safety factor limit as possible.
It is an object of the current invention to yet further optimise the continuous variable transmission per se as indicated by the latter disclosure, thereby maintaining a lowest possible risk of sudden loss of transmission efficiency through the virtually immediate drop of transmission efficiency from optimum to zero at any occurrence of slip due to insufficient clamping force. Simultaneously it is also an object to favourably optimise efficient use of the same transmission.
With a transmission provided with the control measures according to the invention a significant reduction of power (fuel) consumption can be realised at use of the transmission e.g. in a vehicle, based on the insight that in use of such transmission, transmission ratios near overdrive ratio (OD) dominate, i.e. occur for most of the time of use. Thus a relatively insignificant further reduction of the safety factor realises a significant reduction of costs at use of such transmission. This is realised by favourably taking advantage of the circumstance that at higher levels of torque be transmitted, the absolute amount of clamping force has been brought to a significant high level, such that according to the invention any changes in torque to be transmitted at such levels of actual torque, either initiated through the engine or through road circumstances, become relatively small at increasing actual torque levels. In this manner it is provided that e.g. at the highest torque transmittable by the relevant transmission, the safety factor may be further reduced to close to 1. The invention is also based upon the underlying insight and finding that, whereas in so called LOW transmission mode the slip rate of a transmission gradually increases with increasing torques to be transmitted and shows a jump in slip rate within the torque range transmittable, whereas the slip rate at overdrive reliably remains at a relatively low level before or it suddenly increases into infinite maximum torque transmittable by the relevant transmission.
Favourable embodiments of the transmission and control thereof are provided below.